| Message ID | 20190927234008.11513-4-leonardo@linux.ibm.com |
|---|---|
| State | Changes Requested |
| Headers | show |
| Series | Introduces new count-based method for monitoring lockless pagetable walks | expand |
On Fri, Sep 27, 2019 at 08:40:00PM -0300, Leonardo Bras wrote:
> As decribed, gup_pgd_range is a lockless pagetable walk. So, in order to
^ typo
On Mon, 2019-09-30 at 14:09 +0300, Kirill A. Shutemov wrote: > On Fri, Sep 27, 2019 at 08:40:00PM -0300, Leonardo Bras wrote: > > As decribed, gup_pgd_range is a lockless pagetable walk. So, in order to > ^ typo > Fixed, thanks!
On Mon, 2019-09-30 at 14:51 -0700, John Hubbard wrote: > On 9/27/19 4:40 PM, Leonardo Bras wrote: > > As decribed, gup_pgd_range is a lockless pagetable walk. So, in order to > > monitor against THP split/collapse with the couting method, it's necessary > > s/couting/counting/ > Thanks, fixed for v5. > > to bound it with {start,end}_lockless_pgtbl_walk. > > > > There are dummy functions, so it is not going to add any overhead on archs > > that don't use this method. > > > > Signed-off-by: Leonardo Bras <leonardo@linux.ibm.com> > > --- > > mm/gup.c | 8 ++++++++ > > 1 file changed, 8 insertions(+) > > > > diff --git a/mm/gup.c b/mm/gup.c > > index 98f13ab37bac..7105c829cf44 100644 > > --- a/mm/gup.c > > +++ b/mm/gup.c > > @@ -2325,6 +2325,7 @@ static bool gup_fast_permitted(unsigned long start, unsigned long end) > > int __get_user_pages_fast(unsigned long start, int nr_pages, int write, > > struct page **pages) > > { > > + struct mm_struct *mm; > > I don't think that this local variable adds any value, so let's not use it. > Similar point in a few other patches too. It avoids 1 deference of current->mm, it's a little performance gain. > > > unsigned long len, end; > > unsigned long flags; > > int nr = 0; > > @@ -2352,9 +2353,12 @@ int __get_user_pages_fast(unsigned long start, int nr_pages, int write, > > > > if (IS_ENABLED(CONFIG_HAVE_FAST_GUP) && > > gup_fast_permitted(start, end)) { > > + mm = current->mm; > > + start_lockless_pgtbl_walk(mm); > > local_irq_save(flags); > > gup_pgd_range(start, end, write ? FOLL_WRITE : 0, pages, &nr); > > local_irq_restore(flags); > > + end_lockless_pgtbl_walk(mm); > > } > > > > return nr; > > @@ -2404,6 +2408,7 @@ int get_user_pages_fast(unsigned long start, int nr_pages, > > unsigned int gup_flags, struct page **pages) > > { > > unsigned long addr, len, end; > > + struct mm_struct *mm; > > Same here. > > > int nr = 0, ret = 0; > > > > if (WARN_ON_ONCE(gup_flags & ~(FOLL_WRITE | FOLL_LONGTERM))) > > @@ -2421,9 +2426,12 @@ int get_user_pages_fast(unsigned long start, int nr_pages, > > > > if (IS_ENABLED(CONFIG_HAVE_FAST_GUP) && > > gup_fast_permitted(start, end)) { > > + mm = current->mm; > > + start_lockless_pgtbl_walk(mm); > > Minor: I'd like to rename this register_lockless_pgtable_walker(). > > > local_irq_disable(); > > gup_pgd_range(addr, end, gup_flags, pages, &nr); > > local_irq_enable(); > > + end_lockless_pgtbl_walk(mm); > > ...and deregister_lockless_pgtable_walker(). > I have no problem changing the name, but I don't register/deregister are good terms for this. I would rather use start/finish, begin/end, and so on. Register sounds like something more complicated than what we are trying to achieve here. > > thanks, Thank you!
On Tue, 2019-10-01 at 12:04 -0700, John Hubbard wrote: > On 10/1/19 10:56 AM, Leonardo Bras wrote: > > On Mon, 2019-09-30 at 14:51 -0700, John Hubbard wrote: > > > On 9/27/19 4:40 PM, Leonardo Bras wrote: > ... > > > > diff --git a/mm/gup.c b/mm/gup.c > > > > index 98f13ab37bac..7105c829cf44 100644 > > > > --- a/mm/gup.c > > > > +++ b/mm/gup.c > > > > @@ -2325,6 +2325,7 @@ static bool gup_fast_permitted(unsigned long start, unsigned long end) > > > > int __get_user_pages_fast(unsigned long start, int nr_pages, int write, > > > > struct page **pages) > > > > { > > > > + struct mm_struct *mm; > > > > > > I don't think that this local variable adds any value, so let's not use it. > > > Similar point in a few other patches too. > > > > It avoids 1 deference of current->mm, it's a little performance gain. > > > > No, it isn't. :) > > Longer answer: at this level (by which I mean, "wrote the C code, haven't looked > at the generated asm yet, and haven't done a direct perf test yet"), none of us > C programmers are entitled to imagine that we can second guess both the compiler > and the CPU well enough to claim that declaring a local pointer variable on the > stack will even *affect* performance, much less know which way it will go! > I did this based on how costly can be 'current', and I could notice reduction in assembly size most of the time. (powerpc) But I get what you mean, maybe the (possible) performance gain don't worth the extra work. > The compiler at -O2 will *absolutely* optimize away any local variables that > it doesn't need. > > And that leads to how kernel programmers routinely decide about that kind of > variable: "does the variable's added clarity compensate for the extra visual > noise and for the need to manage the variable?" That's a good way to decide it. :) > > Here, and in most (all?) other points in the patchset where you've added an > mm local variable, the answer is no. > Well, IMHO it's cleaner that way. But I get that other people may disagree. > > ... start_lockless_pgtbl_walk(mm); > > > Minor: I'd like to rename this register_lockless_pgtable_walker(). > > > > > > > local_irq_disable(); > > > > gup_pgd_range(addr, end, gup_flags, pages, &nr); > > > > local_irq_enable(); > > > > + end_lockless_pgtbl_walk(mm); > > > > > > ...and deregister_lockless_pgtable_walker(). > > > > > > > I have no problem changing the name, but I don't register/deregister > > are good terms for this. > > > > I would rather use start/finish, begin/end, and so on. Register sounds > > like something more complicated than what we are trying to achieve > > here. > > > > OK, well, I don't want to bikeshed on naming more than I usually do, and > what you have is reasonable, so I'll leave that alone. :) > > thanks, Thank for the feedback,
diff --git a/mm/gup.c b/mm/gup.c index 98f13ab37bac..7105c829cf44 100644 --- a/mm/gup.c +++ b/mm/gup.c @@ -2325,6 +2325,7 @@ static bool gup_fast_permitted(unsigned long start, unsigned long end) int __get_user_pages_fast(unsigned long start, int nr_pages, int write, struct page **pages) { + struct mm_struct *mm; unsigned long len, end; unsigned long flags; int nr = 0; @@ -2352,9 +2353,12 @@ int __get_user_pages_fast(unsigned long start, int nr_pages, int write, if (IS_ENABLED(CONFIG_HAVE_FAST_GUP) && gup_fast_permitted(start, end)) { + mm = current->mm; + start_lockless_pgtbl_walk(mm); local_irq_save(flags); gup_pgd_range(start, end, write ? FOLL_WRITE : 0, pages, &nr); local_irq_restore(flags); + end_lockless_pgtbl_walk(mm); } return nr; @@ -2404,6 +2408,7 @@ int get_user_pages_fast(unsigned long start, int nr_pages, unsigned int gup_flags, struct page **pages) { unsigned long addr, len, end; + struct mm_struct *mm; int nr = 0, ret = 0; if (WARN_ON_ONCE(gup_flags & ~(FOLL_WRITE | FOLL_LONGTERM))) @@ -2421,9 +2426,12 @@ int get_user_pages_fast(unsigned long start, int nr_pages, if (IS_ENABLED(CONFIG_HAVE_FAST_GUP) && gup_fast_permitted(start, end)) { + mm = current->mm; + start_lockless_pgtbl_walk(mm); local_irq_disable(); gup_pgd_range(addr, end, gup_flags, pages, &nr); local_irq_enable(); + end_lockless_pgtbl_walk(mm); ret = nr; }
As decribed, gup_pgd_range is a lockless pagetable walk. So, in order to monitor against THP split/collapse with the couting method, it's necessary to bound it with {start,end}_lockless_pgtbl_walk. There are dummy functions, so it is not going to add any overhead on archs that don't use this method. Signed-off-by: Leonardo Bras <leonardo@linux.ibm.com> --- mm/gup.c | 8 ++++++++ 1 file changed, 8 insertions(+)